Laser Apparatuses With Large-Number Multi-Reflection Pump Systems

ABSTRACT

A large number of passes of pump light through an active mirror in a solid state disk laser is realized using a pair of coupled imaging systems, where the optical axes of imaging systems are not coincident. Two imaging systems are optically coupled, so that an image of the first imaging system is an object of the second imaging system, and vice versa. An active mirror is disposed at the object or image plane, or at the focal plane of any one of the coupled imaging systems, where the position of the reflected pump beam during the multi-reflection between the first and second imaging systems is substantially unchanged.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation in part of application Ser. No.11/376,792 filed Mar. 15, 2006, which is a continuation of provisionalapplication No. 60/662,922 filed Mar. 16, 2005, now abandoned, thedisclosures of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a pump system for a solid state laser,or more specifically, to a pump system that provides a large number ofpasses of the pump light through an active mirror with simple opticalmeans.

BACKGROUND OF THE INVENTION

Currently, various lasers have been available for various applications,ranging from low power semiconductor lasers for opto-electronic devicesto high power solid state lasers for manufacturing, which is kin to thefirst laser invented by Maiman in 1967 (U.S. Pat. No. 3,353,115). Incontrast to other light sources that emit incoherent light, a laseremits a coherent light beam. Theoretically, a coherent light beam can befocused in an area having a diameter of substantially the same order asthe wavelength of light to produce high density of energy. The highquality of coherent light beam is typically expressed in a propagationfactor M²=1.

For example, a solid state laser 100 is shown in FIG. 1, which typicallycomprises a solid state crystal rod 102 surrounded by a helicalflashlamp 104. Light emitted by the flashlamp 104, known as pump light,is absorbed by the crystal 102 to excite the electrons in the crystal toan upper energy level. The excited electrons then return to the originalenergy level through an intermediate level resulting in light emissionwith a specific wavelength characterizing the crystal. The crystal rod102 and the flashlamp 104 are configured in an optical cavity formed bya mirror 106 and a partial mirror 108. The resonance of the cavity andthe stimulated emission in the crystal result in the emission of acoherent laser light beam 110 through the partial mirror 108.

The main merit of a solid state laser is its high power beam. Thecrystal rod absorbs pump light supplied by the flashlamp to its side andtransforms it into a high power laser beam emitted from its face. Alonger crystal rod will absorb more pump energy and thus emit a higherpower laser beam.

However, since the laser beam travels through the crystal rod manytimes, the quality of light beam, e.g., the regularity of its wavefront,will be degraded by the inhomogeneity of the rod including the uneventhermal distribution in the rod. The quality of a laser beam isreflected in its focusability. For a perfect laser beam, the beampropagation factor M² is 1. For example, the beam propagation factor M²of a multi kilo-Watt solid state rod laser may be larger than 150,meaning the focusability of the beam is 150 times worse than thetheoretical limit (M²=1).

A logical solution to less degradation of beam quality (i.e., smallerM²) would be shortening the crystal rod, which shortens the light pathinside the rod. When the rod is getting shorter, it eventually becomes adisk. A further logical solution would be illuminating the face of thedisk with the pump light instead of illuminating the side of the disk,since the face has much larger area than the side of the disk to receivethe pump light.

For example, a solid state disk laser was disclosed in U.S. Pat. No.5,553,088 (1996) to Brauch et al. The disclosed solid state disk laser200 is shown in FIG. 2, which comprises a crystal disk 202 mounted on aheat sink 204 where a reflective layer 206 is disposed between the disk202 and the heat sink 204. A face 208 of the disk 202 opposite to theheat sink 204 is AR (anti reflection) coated. The disk assemblyincluding a disk 202, a heat sink 204, a reflective layer 206, and an ARcoating 208 is referred to as an active mirror.

A diverging pump light 210 exiting from a light delivery device 212 suchas a fiber bundle is focused on the disk 202 by a lens 214. The pumplight can be provided by a laser diode or a set of laser diodes (notshown). The pump light 210 is incident obliquely on the disk 202. Thepump light passes through the AR coated face 208 and the disk 202, andis reflected by the reflective layer 206 to pass the disk for the secondtime. The reflected pump light 210 is focused by a lens 216 on a mirror218. The pump light is then reflected by the mirror 218 and passes thedisk 202. After the pump light is reflected by the layer 206, it passesthe disk 202 again, and returns to the light delivery device 212.

An optical cavity is formed by a mirror 220, a partial mirror 222, andthe reflective layer 206 on the back of the disk 202 to generate a laserbeam 224 oblique to the disk 202. In this way, the disk 202 is in thecavity and multiple passes of the laser beam 224 through the disk 202are realized. The laser beam 224 exits from the partial mirror 222.

In this example, 4 passes of the pump light through the disk aredemonstrated. A similar pump system that provides 4 passes of the pumplight through an active mirror is also taught in US Patent ApplicationPublication No. 2005/0152415 to Giesen et al. Although more passes ofpump light through the disk are required to produce a higher power laserbeam, all disclosed methods can only provide limited numbers of passesof pump light through the disk.

Another method disclosed in the same U.S. Pat. No. 5,553,088 (1996) usesfour spherical mirrors and one plane mirror disposed next to the crystaldisk to provide 8 passes of the pump light through the disk. Forexample, a pump system for generating multi-pass pump light 300 is shownin FIG. 3, which comprises four individual spherical mirrors 302, 304,306, and 308. A diverging pump light 310 exiting from a light deliverydevice 312 is focused by mirror 302 on a disk assembly, which is anactive mirror 314. The active mirror 314 comprises a plane crystal disk,a heat sink, a reflective layer between the disk and the heat sink, andan AR coated face of the disk opposite to the heat sink. The pump lightis reflected by the active mirror 314 to mirror 306. Mirror 306 focusesthe light on a plane mirror 316 disposed next to the active mirror 314.Plane mirror 316 reflects the light to mirror 308. Mirror 308 reflectsthe light to the active mirror 314. The active mirror 314 reflects thelight to mirror 304. Mirror 304 reflects the light back to the activemirror 314, and reverses the whole light path.

The light path is as follows. Light delivery device 312→(1) sphericalmirror 302→(2) active mirror 314→(3) spherical mirror 306→(4) planemirror 316→(5) spherical mirror 308→(6) active mirror 314→(7) sphericalmirror 304→(8) active mirror 314→(9) spherical mirror 308→(10) planemirror 316→(11) spherical mirror 306→(12) active mirror 314→(13)spherical mirror 302→(14) light delivery device 312. The pump light hitsthe active mirror 4 times at steps (2), (6), (8), and (12). Since eachhit produces two passes, 8 passes of the pump light through the activemirror are realized.

The disadvantages of this method are: (1) the number of passes of thepump light through the active mirror is limited by the number of theindividual spherical mirrors (e.g., 4 individual spherical mirrorsprovide 8 passes), (2) the number of the individual spherical mirrors islimited by the size of the mirror, and (3) the mechanical system forsupporting a plurality of individual spherical mirrors is complex andcostly.

Another approach was disclosed by Stewen et al. (C. Stewen, K. Contag,M. Larionov, A. Giesen, and H. Hugel, A 1-kW cw thin disc laser, IEEE J.Selected Topics in Quant. Elect. Vol. 6, 650-657, 2000) as shown in FIG.4( a). A diverging pump light exiting from a light delivery device 412such as a fiber bundle is collimated by a lens 414. The collimated pumplight is incident on a segment 401 of a parabolic mirror 416, which hasa central hole for allowing a laser beam (not shown) generated from anactive mirror 418 to get through the mirror 416.

The pump light is focused by the parabolic mirror to the active mirror418. The active mirror 418 reflects the light to a segment 402 of themirror 416. The mirror collimates and reflects the light to a foldingmirror 420. The folding mirror translates and reflects the collimatedbeam to a segment 403 of the mirror 416.

Further referring to FIG. 4( b), at segment 403, the mirror reflects thelight to the active mirror 418. The active mirror reflects it to asegment 404. The mirror reflects it to a second folding mirror (notshown). The second folding mirror translates and reflects it to asegment 405. The mirror reflects it to the active mirror 418. The activemirror reflects it to a segment 406. The mirror reflects it to a thirdfolding mirror (not shown). The third folding mirror translates andreflects it to a segment 407. The mirror reflects it to the activemirror 418. The active mirror 418 reflects it to a segment 408. Themirror reflects it to a plane mirror (not shown). The plane mirrorreflects it to segment 408, and the pump light reverses its light path,until it is reflected by the mirror at segment 401 toward the device412. In this way, the pump light hits the active mirror 418 for 8 times.Since each hit produces two passes, 16 passes of the pump light throughthe active mirror are realized. Cross-section 422 in FIG. 4( b) is shownin FIG. 4( c). FIG. 4( a) corresponds to cross-section 424 in FIG. 4(b).

Similarly, the disadvantages of this method are: (1) the number ofpasses of the pump light through the active mirror is limited by thenumber of the folding mirrors (e.g., 3 folding mirrors provide 16passes), (2) the number of the folding mirrors is limited by the size ofthe parabolic mirror and the folding mirror, and (3) the mechanicalsystem for supporting a plurality of folding mirrors is complex andcostly.

Similar methods using a lens or a mirror together with a number ofdiscrete prisms to direct the pump beam back to an active mirror weredisclosed in U.S. Pat. No. 6,778,580 (2004) to Erhard and Giesen.Accordingly, the disadvantages of these methods include: (1) the numberof passes of the pump light through the active mirror is limited by thenumber of prisms, (2) the number of prisms is limited by the size of theprism, and (3) the mechanical system for supporting a plurality ofprisms is complex and costly.

Therefore, the main disadvantage of prior-art pump systems is that onlya limited number of passes of the pump light through the active mirrorcan be achieved with great difficulty. For example:

-   -   a system using an active mirror and a mirror (total 2        components) can provide 4 passes of the pump light through the        active mirror;    -   a system using an active mirror, a plane mirror, and 4 spherical        mirrors (total 6 components) can provide 8 passes of the pump        light through the active mirror;    -   a system using an active mirror, a parabolic mirror, 3 folding        mirrors, and a plane mirror (total 6 components) can provide 16        passes of the pump light through the active mirror.        Accordingly, better methods for providing larger number of        passes of the pump light through the active mirror with simplest        possible optical means are desired.

BRIEF SUMMARY OF THE INVENTION

Therefore, the present invention has been made in view of theabove-mentioned disadvantages, and it is an object of the presentinvention to provide methods and apparatuses for allowing a large numberof passes of the pump light through the active mirror with simplestpossible optical means.

According to one aspect of the present invention, a large number ofpasses of the pump light through the active mirror can be realized usinga pair of coupled imaging systems, where the optical axes of imagingsystems are not coincident. Two imaging systems are optically coupled,so that a point A′, which is a focused pump beam, is imaged at a pointB′ by the first imaging system, and point B′ is imaged back at point A′by the second imaging system. The optical axes of two imaging systemsare not coincident, so that the reflected pump beam changes itsdirection during the multi-reflection between the first and secondimaging systems, although points A′ and B′ are substantially unchanged.At least one of the two points A′ and B′ is in an active mirror.

According to another aspect of the present invention, a large number ofpasses of the pump light through the active mirror can be realized usinga pair of coupled imaging systems, where the optical axes of imagingsystems are not coincident. Two imaging systems are optically coupled,so that a point 1, which is a focused pump beam, is imaged to a point 2by the first imaging system, point 2 is imaged to a point 3 by thesecond imaging system, point 3 is imaged to a point 4 by the firstimaging system, and so on. The optical axes of two systems are notcoincident, so that points 1 and 3 are not coincident. Similarly, points2 and 4 are not coincident, and so on. The focused pump beam iscollimated by a lens, and the collimated pump beam is reflected by anactive mirror disposed at a focal plane of the lens. The position of thereflected collimated beam at the active mirror is substantiallyunchanged during the multi-reflection between the first and secondimaging systems.

The details of one or more embodiments of the subject matter describedin this specification are set forth in the accompanying drawings and thedescription below. Other features, aspects, and advantages of thesubject matter will become apparent from the description, the drawings,and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the first solid state laser;

FIG. 2 shows a prior-art solid state disk laser that provides 4 passesof the pump light through an active mirror;

FIG. 3 shows a prior-art solid state disk laser that provides 8 passesof the pump light through an active mirror;

FIG. 4( a) shows a prior-art solid state disk laser that provides 16passes of the pump light through an active mirror;

FIG. 4( b) shows positions of the pump beam on the parabolic mirror ofFIG. 4( a);

FIG. 4( c) shows a cross-section view of FIG. 4( b) showing a foldingmirror that translates a collimated beam;

FIG. 5 shows an embodiment of a multi-reflection pump system including alarge spherical mirror and two small spherical mirrors;

FIG. 6 shows an embodiment of a laser system using the pump system ofFIG. 5;

FIG. 7 shows an embodiment of a position of the pump beam on the largespherical mirror of FIG. 5 during multi-reflection;

FIG. 8 shows an embodiment of a spherical active mirror of FIG. 5;

FIG. 9 shows an embodiment of a position of the pump beam on the largespherical mirror of FIG. 5 during multi-reflection;

FIG. 10 shows an embodiment of a beam conversion means to convert anexit beam into an incident beam;

FIG. 1 shows positions of the pump beam on the large spherical mirror ofFIG. 5 during multi-reflection;

FIG. 12 shows an embodiment of a multi-reflection pump system includinga lens, a plane active mirror, and a roof prism;

FIG. 13 shows positions of the focused light at the stop plane of FIG.12 during multi-reflection;

FIG. 14( a) shows an embodiment of a multi-reflection pump systemincluding a lens, a plane active mirror, and a corner cube prism;

FIG. 14( b) shows positions of the focused light at the stop plane ofFIG. 14( a) during multi-reflection;

FIG. 15 shows an embodiment of a multi-reflection pump system includingtwo lenses and two plane mirrors;

FIG. 16 shows an embodiment of a multi-reflection pump system includingtwo parabolic mirrors and two plane mirrors;

FIG. 17 shows an embodiment of a multi-reflection pump system includinga lens, two plane mirrors, and a plane active mirror; and

FIG. 18 shows an embodiment of a laser amplifier using the pump systemof FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION Multi-Reflection System UsingThree Spherical Mirrors

FIG. 5 shows one embodiment of pump system in which a large number ofpasses of the pump light through the active mirror can be realized. Thepump system 500 comprises a large concave spherical mirror 502 and twosmall concave spherical mirrors 504 and 506. In an implementation, oneof the small mirrors can be a simple mirror (not an active mirror) andthe other one of the small mirrors can be an active mirror. In anotherimplementation, both small mirrors can be active mirrors.

For example, all mirrors have the same radii R. The center of curvatureof the large spherical mirror 502 is located at point O, which is thecentral point between small spherical mirrors 504 and 506. The centersof curvature of spherical mirrors 504 and 506 are located at points Aand B, respectively, on the large spherical mirror 502. The arc betweenA and B is d degree. All points A, B and O are in a meridional plane. Inother words, the plane of paper of FIG. 5 is a meridional plane. Themeridional plane crosses the mirror 502 at a meridional line.

A beam 1 entering the pump system 500 strikes mirror 504 at point A′.Beam 1 is reflected by mirror 504 into a beam 2 toward mirror 502. Beam2 is further reflected by mirror 502 into a beam 3 toward mirror 506.Beam 3 strikes mirror 506 at point B′. Beam 3 is then reflected into abeam 4 toward mirror 502. Beam 4 is then reflected by mirror 502 into abeam 5, which strikes mirror 504 substantially at A′. In other words,the pump light beam strikes substantially the same location at mirror504. At this step, one reflection cycle is completed. Beam 5 has shiftedfrom beam 1 by an arc about 2d degree on mirror 502, where d is the arcbetween points A and B on mirror 502.

The next reflection cycle begins when beam 5 is further reflected bymirror 504 into a beam 6, and ends when a beam 9 strikes mirror 504substantially at A′. At the end of each reflection cycle, the positionof the reflected beam on mirror 502 shifts by an arc of 2d on mirror502. The position of the end-of-cycle beam (e.g., beams 1, 5, 9, 13, . .. , 41) shifts from a first edge of mirror 502 where beam 1 enters thesystem to the central portion of mirror 502 and further to the otheredge of the mirror. Eventually a reflected beam 42 is out of the mirror,for example from the first edge of the mirror, and leaves the pumpsystem 500.

The number of reflections depends on the ratio of the arc of mirror 502to d (arc between A and B). On the other hand, d is determined by thenumerical aperture (NA) of the incoming pump beam, which will bedescribed later.

FIG. 6 shows an example of laser system 600 using a pump system of FIG.5, where a laser beam 614 is in the meridional plane of the system 600.The pump system comprises a large concave spherical mirror 602 and twosmall concave spherical active mirrors 604 and 606. An active mirrortypically comprises a laser crystal disk mounted on a heat sink, ahigh-reflection layer between the laser crystal disk and the heat sink,and an AR coated front surface. The high-reflective coated surface ismounted on a common heat sink 608. It is also possible to use twoseparate heat sinks instead of a common heat sink.

The center of curvature of the large spherical mirror 602 is located atpoint O, which is the central point between spherical active mirrors 604and 606. The centers of curvature of spherical active mirrors 604 and606 are located at points A and B, respectively, on the sphere of themirror 602. A hole is made at the central region of mirror 602 to allowa laser beam 614 to pass through the hole. The laser system 600 alsocomprises a high-reflection mirror 610 and a partial-reflection mirror612, which together with active mirrors 604 and 606 form a laser cavitythrough the hole of mirror 602. A laser beam 614 is generated in themeridional plane (the plane of A, B, and O), which is the plane of paperof FIG. 6. The laser beam 614 is emitted through the partial-reflectionmirror 612.

Multiple modules comprising a spherical mirror and two active mirrorsmentioned above can be put in series in a laser cavity to form a laserof much higher output power.

A light delivering device 616, for example a fiber or fiber bundle,emits diverging pump light 618 originated from a laser diode or otherpump light source 620. The pump light 618 is focused by a lens 622 at A′on active mirror 604. The incoming pump beam to A′ is a light cone withits tip at A′ and its base on lens 622. The light cone is representedwith beam 1 in FIG. 5.

Referring to FIG. 5, beam 1 is an incoming pump light that is focused atpoint A′. Beam 1 is associated with a light cone where its tip is at A′,and its base is on the sphere of mirror 502. A′ is then imaged at B′ bymirrors 504 and 502, B′ is imaged back at A′ by mirrors 506 and 502, A′is again imaged at B′ by mirrors 504 and 502, B′ is imaged back at A′ bymirrors 506 and 502, . . . , and so on. Although the positions of imagesA′ and B′ are substantially unchanged, the reflected beam strikes mirror502 at a different location for each reflection. In other words, thetips of light cones associated with beams 1, 2, and 5 are at A′ buttheir bases on the sphere of mirror 502 are moved. Similarly, the tipsof light cones associated with beams 3 and 4 are at B′ but their baseson mirror 502 are moved. Eventually the base of the light cone is out ofthe mirror and thus the reflected beam leaves the mirror. For example,the beam leaves mirror 502 at a location near the location where thepump beam enters the multi-reflection system.

For example, if the arc AB is 5°, the arc between beam 1 and beam 5 onmirror 502 is 10°. Beam 1 is a light cone. The tip of the light cone isat A′ and the base of the light cone is on mirror 502. For example, thediameter of the base of the light cone equals to the arc between beam 1and beam 5 on mirror 502 or 10°. To allow the incident pump light, e.g.,beam 1, to fully strike mirror 504, the light cone must not be blockedby mirror 502 (see FIG. 6). If mirror 502 is a 100° arc, the mirror 502covers 10 10° arcs, and the reflected light can strike mirrors 504 and506 for 11 and 10 times, respectively. When beam 1 is in the meridionalplane, all reflected beams are also in the meridional plane as shown inFIG. 5.

While FIG. 5 shows a side view of mirror 502, FIG. 7 shows a front viewof mirror 502. FIG. 7 shows a configuration that beam 1 is not in themeridional plane, but it still strikes at A′ of small mirror 504 (notshown in FIG. 7). Accordingly, all positions of reflected beams onmirror 502, i.e., bases of light cones on mirror 502, are on two linessymmetrical with respect to the meridional line. The distance betweeneach two neighboring points (e.g., beams 1 and 5) is about 2d.

Similar to FIG. 5, the beams gradually shift to the central portion ofmirror 502 and then to the edge. At least an edge of mirror 502 istrimmed for beam input and output. The shadowed squares indicate theincident and exit positions of beams. Each of the squares shows thereflected beams on mirror 502, e.g., the base of light cone, during themulti-reflection process. The base of the light cone can be any shape,for example, circular, elliptical, square, . . . , which is shaped by anaperture. For example, a square-shaped base is shown.

At the shadowed area for the exit beam, an additional small sphericalmirror with its center of curvature located at A′ of mirror 504 can beused to reflect the exit beam (beam 45) back to mirror 504 and reversethe light along the original path all the way back to the entranceposition (beam 1). Therefore, when the incident beam (beam 1) is not inthe meridional plane, the number of reflections can be doubled by addinga mirror to reverse the light path. In the configuration shown in FIG.7, beams 1, 5, 9, 13, 17, 21, 25, 29, 33, 37, 41, 45, 41, 37, 33, 29,25, 21, 17, 13, 9, 5, hit A′ on mirror 504 for a total of 22 times.Thus, the pump beam can hit mirror 504 for 22 times and mirror 506 for21 times. Since each hit produces two passes, 44 and 42 passes of thepump light through respective active mirrors are realized. Furthermore,since both active mirrors are used to generate a laser beam, theconfiguration shown in FIG. 7 is effectively equivalent to 86 passes ofthe pump beam through an active mirror.

Accordingly, a system using a spherical mirror, two spherical activemirrors, and an extra spherical mirror (total 4 components) can provide44 and 42 passes of the pump light through respective active mirrors,which is effectively equivalent to 86 passes of the pump beam through anactive mirror.

Since there are no reflected beams in the central region of mirror 502,a hole 508 can be made in the central region of mirror 502. Hole 508 isneeded to allow a laser beam passing through mirror 502 (see FIG. 6).

FIG. 8 shows an example of spherical active mirror 800 comprising a thindisk 802 made of a crystal or ceramic, such as Yb:YAG, Nd:YAG, Nd:YVO4or other materials doped with active species. The front and backsurfaces of the disc are both spherical, with the back face coated witha high-reflection layer 804. The disk 802 with the high-reflection layer804 is mounted on a heat sink 810. The disc 802 can also have asphericsurfaces.

The front surface of the disk 802 is covered by a cap 806. The cap 806is made of a transparent material with high heat conductivity, which canbe selected from sapphire, YAG, or diamond. It is preferred that the cap806 contacts with a heat sink (not shown) for heat removal. Onepossibility is to make the cap 806 larger than the disk 802 so the cap806 can be in contact with a heat sink (not shown). For example, the cap806 can be bonded with the disk 802 such as by sintering. The frontsurface of the cap 806 is coated with an AR coating 808. An activemirror can also be made of a plane thin disk or thin film doped withactive species. However, a curved active mirror reduces more ASE(amplified spontaneous emission) noise as compared with a plane activemirror.

In an implementation disclosed above, a laser beam is in the meridionalplane. Accordingly, a hole in the central region of mirror 502 or 602 onthe meridional line is needed to allow the laser beam passing throughthe mirror. In this implementation, the pump beam must not be in thecentral region of the mirror, and must not be on the meridional line. Insome implementations, the laser beam may not be in the meridional plane.Furthermore, the laser beam may not pass through the central region ofthe mirror. Accordingly, the pump beam can be on the meridional line.

FIG. 9 shows a configuration where the pump beam is passing through themeridional line. This configuration allows the pump beam to hit eachmirror 504 and 506 for 33 times. For simplicity and clarity of drawing,the numbers in FIG. 9 indicate the positions of the beam on mirror 502instead of beam numbers. The pump beam is incident from position 1,reflected to position 2, reflected to position 3, . . . , and so on.After multiple reflections, a beam reflected by mirror 504 exits fromposition 22. A beam conversion means can be used to convert the exitbeam from position 22 into a beam incident from position 23 in themeridional plane toward point A′ on mirror 504. Position 33 iscoincident with point A, which is the center of curvature of mirror 504.In an implementation, the beam is reflected by mirror 506 (point B′) toposition 33 on mirror 502, which is the center of curvature of mirror504 (point A). The beam will be reflected from position 33 (point A) tomirror 504 (point A′), then the beam is reflected from point A′ toposition 33 (point A) and back along the original path to position 1.

In this configuration, beams at positions 1, 3, 5, 7, 9, 11, 13, 15, 17,19, 21, 23, 25, 27, 29, 31, 33, 31, 29, 27, 25, 23, 21, 19, 17, 15, 13,11, 9, 7, 5, 3, 1 hit A′ on mirror 504 for a total of 33 times.Similarly, beams at positions 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22,24, 26, 28, 30, 32, 33, 32, 30, 28, 26, 24, 22, 20, 18, 16, 14, 12, 10,8, 6, 4, 2 hit B′ on mirror 506 for 33 times.

Since each hit produces two passes, 66 passes of the pump light througheach active mirror are realized. Furthermore, since both active mirrorsare used to generate a laser beam, the configuration shown in FIG. 9 iseffectively equivalent to 132 passes of the pump beam through an activemirror.

Out of many possibilities, FIG. 10 shows a beam conversion means 1000that can convert the exit beam from position 22 into a beam incidentfrom position 23 in FIG. 9. The beam conversion means 1000 comprises afirst lens 1002, a second lens 1004, and a prism 1006. The first lens1002 collimates the beam at position 22 reflected from A′ of mirror 504(FIG. 9), which is a light cone with tip at A′ and base at position 22.After the collimated beam is translated by prism 1006 from positions 22to 23 and reflected toward A′, the second lens 1004 focuses thecollimated beam back to A′ on mirror 504.

Accordingly, a system using a spherical mirror, two spherical activemirrors, and a beam conversion means (total 4 components) can provide 66passes of the pump light through each active mirror, which iseffectively equivalent to 132 passes of the pump beam through an activemirror. The rest of mirror 502, which is not used for themulti-reflection of pump light, can be trimmed properly to prevent themirror from blocking a laser beam generated by the pump system.

FIG. 11 shows a configuration where the pump beam can be reflected bymirrors 504 for 44 times and 506 for 43 times, provided the beamdiameter at mirror 502 is about 10° arc and the mirror 502 is 100° arc.Similar to a configuration of FIG. 9, a beam conversion means is placedin the path between positions 22 and 23. However, position 23 is not inthe meridional plane. In this way, a pump light beam at position 1initially strikes point A′ of mirror 504. The light beam is eventuallyreflected to position 44 after multiple reflections between mirror 502and mirrors 504 and 506. A small spherical mirror with its center ofcurvature located at A′ of mirror 504 is placed at position 44 toreflect the beam back along its original path to position 1.

Since each hit produces two passes, 88 and 86 passes of the pump lightthrough respective active mirrors are realized. Furthermore, since bothactive mirrors are used to generate a laser beam, the configuration ofFIG. 11 is effectively equivalent to 174 passes of the pump beam throughan active mirror.

Accordingly, a system using a spherical mirror, two spherical activemirrors, a beam conversion means, and an extra spherical mirror (total 5components) can provide 88 and 86 passes of the pump light throughrespective active mirrors, which is effectively equivalent to 174 passesof the pump beam through an active mirror. The number of reflections canbe further increased by adding additional beam conversion means andmirrors. The rest of mirror 502, which is not used for themulti-reflection of pump light, can be trimmed properly to prevent themirror from blocking a laser beam generated by the pump system.

Referring to FIG. 5, the multi-reflection optical system 500 disclosedabove comprises two coupled imaging systems sharing a common mirror 502.The first imaging system includes spherical mirror 502 and sphericalactive mirror 504, which is optically coupled with the second imagingsystem including spherical mirror 502 and spherical active mirror 506.It is possible that one of the active mirrors is a simple mirrorinstead. The first imaging system images point A′ to point B′, and thesecond imaging system images back point B′ to point A′. In other words,the image of the first imaging system is the object of the secondimaging system, and the image of the second imaging system is the objectof the first imaging system. The optical axes of two coupled imagingsystems are not coincident. The first optical axis OA and the secondoptical axis OB form an angle, for example 5°. A pump beam entering themulti-reflection optical system 500 is focused and reflected at A′ onactive mirror 504 and B′ on active mirror 506 during the process ofmulti-reflection.

Multi-Reflection System Using a Lens, a Plane Mirror and a Prism

FIG. 12 shows another embodiment of pump system, 1200, in which a largenumber of passes of the pump light through the active mirror can berealized. The pump system 1200 comprises a lens 1202, a plane activemirror 1204, and a roof prism 1206. Point O is the focal point of thelens 1202 having a focal length f and an optical axis OO′. A planeactive mirror 1204 is disposed in the focal plane of lens 1202 passingthrough the focal point O and perpendicular to the optical axis OO′.Another focal plane of lens 1202 is a stop plane 1208 of the pump system1200. The roof edge of roof prism 1206 is at point P on the stop plane1208. The roof edge of the roof prism 1206 is not on the optical axisOO′ and separated by a distance d′.

A diverging pump light beam is transmitted toward lens 1202 from point 1in the stop plane 1208. Since plane 1208 is a focal plane of lens 1202,the diverging beam transmitted from point 1 will be collimated by lens1202. The collimated beam strikes the active mirror 1204, and is thenreflected by the active mirror. The reflected beam is focused by lens1202 virtually at a point 2 in the stop plane 1208. Points 1 and 2 aresymmetrical with reference to the optical axis OO′. The beam, which isvirtually focused at point 2, is reflected by the prism 1206 into a beamvirtually emitted from point 3 in the stop plane 1208. Points 2 and 3are symmetrical with reference to the roof edge P. After a cycle, point3 will shift by 2d′ relative to point 1.

In other words, point 1 is imaged to point 2 by lens 1202 and the activemirror 1204, point 2 is imaged to point 3 by roof prism 1206, point 3 isimaged to point 4 (not shown) by lens 1202 and the active mirror 1204, .. . , and so on. Accordingly, there are two optically coupled imagingsystems. The first imaging system includes lens 1202 and the activemirror 1204 with an optical axis OO′. The second imaging system is theroof prism 1206 with roof edge P, where P is not on the optical axesOO′. The roof edge P is separated by a distance d′ from the optical axesOO′.

The optical axis OO′ is in a meridional plane, which is perpendicular tothe plane active mirror 1204, and the roof edge of prism 1206. Thus, theplane of paper of FIG. 12 is the meridional plane. The meridional planecrosses the stop plane 1208 at a meridional line.

For example, FIG. 13 shows the position of focused light at the stopplane 1208. The incident beam is from point 1, which is not in themeridional plane. The roof edge of prism 1206 is perpendicular to themeridional line. The distance of O from the roof edge is d′. Asmentioned earlier, points 2 and 1 are symmetrical with reference to theoptical axis OO′ or point O in FIG. 13, points 3 and 2 are symmetricalwith reference to the roof edge, points 4 and 3 are symmetrical to pointO, points 5 and 4 are also symmetrical to the roof edge, . . . , and soon.

Note that FIGS. 7, 9, 11 show the bases of light cones on the sphericalmirror 502. In contrast, in FIG. 13, points 1, 2, 3, . . . indicate thetips of light cones on the stop plane 1208. Furthermore, in the pumpsystem of FIG. 12, the active mirror is struck by collimated pump beams.On the other hand, in the pump system of FIG. 5, pump beams are focusedon the active mirror.

For example, the incident beam from point 1 will be reflected by mirror1204 for 9 times and exits from point 18. The circle in FIG. 13 is theclear aperture of the stop 1208. The dotted rectangle is roof prism1206. The shadowed area 1 with 2d′ width is the aperture of the incidentbeam. If a small plane mirror is placed at point 18 to reflect the beamback along its original path, the beam will return from point 18 topoint 1. In this way, the beam hits mirror 1204 for 18 times. It can beseen that this result is similar to an example shown in FIG. 7, however,the order of beam position in the stop plane is different.

For example, for a 120°×120° stop aperture and a 10°×10° incident beam,the beam hits mirror 1204 for 26 times. Since each hit produces twopasses, 52 passes of the pump light through respective active mirrorsare realized. A 120°×120° stop aperture is corresponding to NA(numerical aperture) of 0.87. Although lens 1202 can be any lightfocusing means including a simple lens, to achieve such large NA, lens1202 can be a combination of lenses, an aspheric lens, a HOE(holographic optical element), or a DOE (diffractive optical element).The roof prism 1206 can also be replaced with two planar mirrorspositioned perpendicular to each other.

FIG. 14( a) shows a pump system 1400 where roof prism 1206 is replacedby a corner cube prism 1406. The positions of beam in the stop plane areshown in FIG. 14( b). Since a roof prism provides a left-to-right mirrorimage and a corner cube prism provides a left-to-right and up-to-downmirror image, the positions of beam in FIGS. 13 and 14( b) aredifferent.

Accordingly, a system using a lens, an active mirror, a roof prism or acorner cube prism, and a plane mirror (total 4 components) can possiblyprovide 52 passes of the pump light through an active mirror. The numberof reflection depends on the NA of the lens.

Referring to FIGS. 12 and 14( a), the multi-reflection optical systems1200 and 1400 disclosed above comprise two coupled imaging systems. Thetwo imaging systems are optically coupled, so that a point 1 is imagedto a point 2 by the first system, point 2 is imaged to a point 3 by thesecond system, point 3 is imaged to a point 4 by the first system, andso on. The optical axes of two systems are parallel and separated by adistance, so that points 1 and 3 are also separated. Similarly, points 2and 4 are separated. However the position of the reflected collimatedbeam at an active mirror is substantially unchanged during the processof multi-reflection.

Other Multi-Reflection System Using Two Coupled Imaging Systems

FIG. 15 shows yet another embodiment of pump system 1500, in which alarge number of passes of the pump light through the active mirror canbe realized. The roof prism 1206 of FIG. 12 is replaced by a lens and amirror. The pump system 1500 comprises a first lens 1502, a plane activemirror 1504, a second lens 1506, and a plane mirror 1508. The planemirror 1508 can also be a plane active mirror. This arrangement showstwo optically coupled imaging systems, the first imaging systemincluding the first lens 1502 and the plane active mirror 1504, and thesecond imaging system including the second lens 1506 and the planemirror 1508.

The focal lengths of lenses 1502 and 1506 are f1 and f2, respectively.The active mirror 1504 is disposed at the focal point O1 of lens 1502,and mirror 1508 is disposed at the focal point O2 of lens 1506,perpendicular to each optical axis. The optical axes of the firstimaging system comprising lens 1502 and the second imaging systemcomprising lens 1506 are parallel and shifted by a distance d′. The stopplane 1510 is the common focal plane of lenses 1502 and 1506.

A diverging pump light beam is transmitted toward lens 1502 from point 1in the stop plane 1510. Point 1 is imaged to a point 2 in the stop plane1510 by the first lens 1502 and the active mirror 1504. Point 2 is thenimaged to a point 3 by the second lens 1506 and mirror 1508. Point 3 isfurther imaged by the first lens 1502 and the active mirror 1504, and soon. Similar to a corner cube prism, the imaging system of lens 1506 andmirror 1508 provides a left-to-right and up-to-down mirror image.Therefore, the positions of beam in the stop plane 1510 are the same asthat shown in FIG. 14( b), by changing O into O1 and P into O2. In atheoretical view, the pump system 1500 is similar to pump systems 1200and 1400 in FIGS. 12 and 14( a), respectively.

FIG. 16 shows further another embodiment of pump system 1600 in which alarge number of passes of the pump light through the active mirror canbe realized. The pump system 1600 comprises a first parabolic mirror1602, a plane active mirror 1604, a second parabolic mirror 1606, and aplane mirror 1608. The plane mirror 1608 can also be a plane activemirror. This arrangement also shows two optically coupled imagingsystems, the first imaging system including the first parabolic mirror1602 and the plane active mirror 1604, and the second imaging systemincluding the second parabolic mirror 1606 and the plane mirror 1608.

Points O3 and O4 are the vertexes of parabolic mirrors 1602 and 1606,respectively. Points O1 and O2 are the focal points of parabolic mirrors1602 and 1606, respectively. The optical axis of the first imagingsystem comprising parabolic mirror 1602 is O3O1. The optical axis of thesecond imaging system comprising parabolic mirror 1606 is O2O4. Twooptical axes are parallel and separated by a distance d′.

The focal point O1 is on the plane active mirror 1604, and the focalpoint O2 is on the plane mirror 1608. The plane active mirror 1604 andmirror 1608 are tilted to allow multi-reflection. An incident pump beam11-12 is focused by a lens (not shown) at the focal point O1 on theactive mirror 1604. The beam is reflected by the active mirror 1604 intoa beam 21-22. The beam 21-22 is reflected by parabolic mirror 1602 intoa beam 31-32 parallel to its optical axis. The beam 31-32 is focused byparabolic mirror 1606 at O2 on mirror 1608 as a beam 41-42. The beam41-42 is further reflected by mirror 1608 into a beam 51-52, which isreflected by parabolic mirror 1606 into a beam 61-62 parallel to itsoptical axis, and then focused at O1 by parabolic mirror 1602 as a beam71-72, completing a cycle.

FIG. 17 shows yet further another embodiment of pump system 1700 inwhich a large number of passes of the pump light through the activemirror can be realized. The pump system 1700 comprises a lens 1702, aplane mirror 1704, a tilted plane active mirror 1706 and a plane mirror1708. The plane mirror 1708 can also be an active mirror. Lens 1702 canbe a combination of lenses, an aspheric lens, a HOE (holographic opticalelement), or a DOE (diffractive optical element). This arrangement showstwo optically coupled imaging systems. The first imaging system includesthe tilted active mirror 1706, lens 1702 and mirror 1704, the secondimaging system includes mirror 1708, lens 1702 and mirror 1704.

The active mirror 1706 and mirror 1708 are disposed in the focal planeof lens 1702. Mirror 1704 is normal to the optical axis OO′ of lens1702, and disposed with a small distance d′ to lens 1702. The normal ofthe active mirror 1706 is tilted to the optical axis OO′ of lens 1702 bya small angle θ/2. Mirror 1708 is tilted relative to the active mirror1706 by a small angle θ. Accordingly, the axes of two imaging systemsform an angle θ. The active mirror 1706 and mirror 1708 are symmetricalwith reference to OO′.

A collimated pump beam is incident from position 1. The collimated beamis focused at A′ on the active mirror 1706. Point A′ is imaged by thefirst imaging system including the active mirror 1706, lens 1702, andmirror 1704 at a point B′ on mirror 1708. Point B′ is then imaged by thesecond imaging system including mirror 1708, lens 1702, and mirror 1704back to A′. After one cycle, the beam position translates a distance ofabout 2d′θ to position 2 on mirror 1704. The imaging process is repeateduntil the pump beam is out of the mirror 1704. In a theoretical view,the pump system 1700 is similar to pump system 500 in FIG. 5.

Therefore, it is understood that a large number of passes of the pumplight through the active mirror can be realized using a pair of coupledimaging systems, where the optical axes of imaging systems are notcoincident. In some implementations, two imaging systems are opticallycoupled, so that a point A′ is imaged at a point B′ by the first imagingsystem, point B′ is imaged back at point A′ by the second imagingsystem. The optical axes of two imaging systems are not coincident, sothat the reflected beam changes its direction for each reflectionalthough the image point A′ and B′ are not substantially changed. Atleast one of the two points A′ and B′ is in an active mirror.

In some implementations, two imaging systems are optically coupled, sothat a point 1 is imaged to a point 2 by the first system, point 2 isimaged to a point 3 by the second system, point 3 is imaged to a point 4by the first system, and so on. The optical axes of two systems are notcoincident, so that points 1 and 3 are not coincident. Similarly, points2 and 4 are not coincident. However the position of the reflectedcollimated beam on an active mirror is substantially unchanged.

Laser Amplifier and Q-switch Lasers

In addition to providing a laser, the pump systems disclosed above canbe used to provide a laser amplifier, where the amplified laser beamtravels multiple times to extract energy from the laser crystal disk.Not only can a multi-reflection system be used for pump light, it canalso be used as a laser amplifier. The pump and amplified beams can usea single multi-reflection system or separate multi-reflection systems.

Unlike a pump beam, an amplified laser beam has a very small divergenceangle, e.g., almost collimated. In order to fully extract the energystored in the active mirrors, the spot size of the amplified beam on anactive mirror is made substantially the same as the spot size of thepump beam on the same active mirror.

For example, FIG. 18 shows a laser amplifier 1800 based on anarrangement shown in FIG. 5 comprising a spherical mirror 1802 and twospherical active mirrors 1804 and 1806. An incoming laser beam 11-12 isfirst focused at point E1. After being focused at point E1, beam 11-12strikes the active mirror 1804, which reflects beam 11-12 into a beam21-22. Beam 21-22 is virtually focused at point E2. Beam 21-22 isreflected by mirror 1802 into a beam 31-32 and focused at point E3. Beam31-32 is further reflected by active mirror 1806 into a beam 41-42, andvirtually focused at point E4. The process is repeated until the beam isout of mirror 1802. The number of reflections can be further increasedusing additional beam conversion means and reflectors as discussedpreviously.

Therefore, it is possible that a laser beam strikes an active mirrorover hundred times in a multi-reflection system. This laser amplifiercan replace regenerative amplifiers that have complicated structures.

In addition to a laser amplifier mentioned above, a multi-reflectionsystem including active mirrors can also be used as an oscillator. Forexample, a pair of cavity mirrors, e.g., a high-reflection mirror and apartial-reflection mirror, are disposed at the beam input and outputpositions (see FIG. 6). Since the gain in the cavity is sufficientlyhigh, a switch or SESAM (semiconductor saturable absorber mirror) can beinserted in the oscillator to form a novel active mirror Q-switch laseror mode-locked laser.

While this specification contains many specifics, these should not beconstrued as limitations on the scope of any invention or of what may beclaimed, but rather as descriptions of features that may be specific toparticular embodiments of particular inventions. Certain features thatare described in this specification in the context of separateembodiments can also be implemented in combination in a singleembodiment. Conversely, various features that are described in thecontext of a single embodiment can also be implemented in multipleembodiments separately or in any suitable subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. Moreover, the separation of various system components in theembodiments described above should not be understood as requiring suchseparation in all embodiments.

Particular embodiments of the subject matter described in thisspecification have been described. Other embodiments are within thescope of the following claims. For example, the actions recited in theclaims can be performed in a different order and still achieve desirableresults. As one example, the processes depicted in the accompanyingfigures do not necessarily require the particular order shown, orsequential order, to achieve desirable results.

1. A multi-reflection pump system comprising: a first imaging systemhaving a first optical axis, a second imaging system having a secondoptical axis, said second optical axis not being coincident with saidfirst optical axis, said first imaging system being optically coupledwith said second imaging system so that an image of said first imagingsystem forms an object of said second imaging system and an image ofsaid second imaging system forms an object of said first imaging system,at least one of said first and second imaging systems including anactive mirror, and a source generating a pump light beam, the pump lightbeam entering said pump system and leaving said pump system aftermultiple reflections between said first imaging system and said secondimaging system, said pump light beam being focused so that it is anobject of said first imaging system, said pump light beam striking saidactive mirror multiple times during said multiple reflections betweensaid first imaging system and said second imaging system.
 2. A systemaccording to claim 1 wherein said first optical axis and said secondoptical axis form an angle between them, wherein said object of saidfirst imaging system being said image of said second imaging system andsaid object of said second imaging system being said image of said firstimaging system are substantially unchanged during said multiplereflections between said first imaging system and said second imagingsystem, and wherein at least one of said object of said first imagesystem and said object of said second image system is at said activemirror.
 3. A system according to claim 1 wherein said first optical axisand said second optical axis are parallel and separated by a distance,wherein said object of said first imaging system being said image ofsaid second imaging system and said object of said second imaging systembeing said image of said first imaging system move during said multiplereflections between said first imaging system and said second imagingsystem, wherein said pump light beam is collimated by a lens of saidfirst imaging system and reflected by said active mirror disposed at afocal plane of said lens, and wherein said collimated pump light beamstrikes and is reflected by said active mirror at substantiallyunchanged position.
 4. A system according to claim 2 wherein said firstimaging system and said second imaging system have a common sphericalmirror, said first imaging system comprises said common spherical mirrorand a first small spherical mirror, said second imaging system comprisessaid common spherical mirror and a second small spherical mirror, andwherein at least one of said first and second small spherical mirrors isan active mirror.
 5. A system according to claim 2 wherein said firstimaging system and said second imaging system have a common lens and acommon mirror, said first imaging system comprises said common lens andmirror and a first planar mirror, said second imaging system comprisessaid common lens and mirror and a second planar mirror, and wherein atleast one of said first and second planar mirrors is an active mirror.6. A system according to claim 5 wherein said lens is an optical elementselected from the group consisting of a simple lens, a combination oflenses, an aspheric lens, a holographic optical element (HOE), and adiffractive optical element (DOE).
 7. A system according to claim 3wherein said first imaging system comprises a lens and a plane activemirror, and said second imaging system comprises a roof prism.
 8. Asystem according to claim 7 wherein said lens is an optical elementselected from the group consisting of a simple lens, a combination oflenses, an aspheric lens, a holographic optical element (HOE), and adiffractive optical element (DOE).
 9. A system according to claim 3wherein said first imaging system comprises a lens and a plane activemirror, and said second imaging system comprises said lens and a cornercube prism.
 10. A system according to claim 9 wherein said lens is anoptical element selected from the group consisting of a simple lens, acombination of lenses, an aspheric lens, a holographic optical element(HOE), and a diffractive optical element (DOE).
 11. A system accordingto claim 3 wherein said first imaging system comprises a first lens anda first plane mirror, said second imaging system comprises a second lensand a second plane mirror, and wherein at least one of said first andsecond plane mirrors is an active mirror.
 12. A system according toclaim 10 wherein said lens is an optical element selected from the groupconsisting of a simple lens, a combination of lenses, an aspheric lens,a holographic optical element (HOE), and a diffractive optical element(DOE).
 13. A system according to claim 1 wherein said first imagingsystem comprises a first parabolic mirror and a first plane mirror, andsaid second imaging system comprises a second parabolic mirror and asecond plane mirror, wherein said object of said first imaging systembeing said image of said second imaging system and said object of saidsecond imaging system being said image of said first imaging system aresubstantially unchanged during said multiple reflections between saidfirst imaging system and said second imaging system, wherein said objectof said first image system is at said first plane mirror and said objectof said second image system is at said second plane mirror, and whereinat least one of said first and second plane mirrors is an active mirror.14. An optical system according to claim 1 wherein said active mirrorcomprises a crystal disk, a high-reflection layer on a back side of saiddisk, and a heat sink mounting said disk.
 15. An optical systemaccording to claim 14 wherein said active mirror further comprises a capcovering a front surface of said crystal disk and wherein said capincludes an anti-reflection coating.
 16. An optical system according toclaim 15 wherein said cap contacts the heat sink.
 17. An optical systemcomprising: a relatively larger concave spherical mirror, a firstrelatively smaller concave spherical mirror and a second relativelysmaller concave spherical mirror, at least one of said first and secondrelatively smaller concave spherical mirrors being an active mirror, acenter of curvature of said relatively larger concave spherical mirrorbeing at the centers of said first and second relatively smaller concavespherical mirrors, the centers of curvature of said first and secondrelatively smaller concave spherical mirrors being at said large concavespherical mirror, and a source generating a pump light beam which entersand leaves said system after multiple reflections between saidrelatively larger mirror and said first and second relatively smallermirrors, said beam being focused and reflected at substantiallyunchanged positions at said first and second relatively smaller concavespherical mirrors, said beam striking and being reflected by saidrelatively larger concave spherical mirror at changing positions.
 18. Anoptical system according to claim 17 further comprising ahigh-reflection mirror and a partially reflecting mirror, saidhigh-reflection and partially reflecting mirrors together with saidfirst and second small concave spherical mirrors forming a laser cavity.19. An optical system according to claim 18 wherein said large concavespherical mirror defines a hole at its central region allowing a laserbeam to pass through said large concave spherical mirror.
 20. An opticalsystem according to claim 17 wherein said active mirror comprises acrystal disk and a high-reflection layer on a back of said disk, saiddisk being mounted on a heat sink.
 21. An optical system according toclaim 20 wherein said active mirror further comprises a cap covering afront surface of said crystal disk, said cap including ananti-reflection coating.
 22. An optical system according to claim 21wherein said cap is in contact with the heat sink.
 23. An optical systemaccording to claim 17 further comprising a laser beam entering saidoptical system which is focused at a point in a space between saidrelatively larger spherical mirror and said first relatively smallerspherical mirror, and leaving said optical system after multiplereflections between said relatively larger mirror and said first andsecond relatively smaller mirrors.
 24. An optical system according toclaim 23 further comprising a high-reflection mirror and a partiallyreflecting mirror.
 25. An optical system according to claim 24 furthercomprising a switch for Q-switching.
 26. An optical system according toclaim 24 further comprising a SESAM (semiconductor saturable absorbermirror) for mode locking.
 27. A method for pumping an active mirrormultiple times comprising: providing a first imaging system having afirst optical axis, providing a second imaging system having a secondoptical axis, said second optical axis being not coincident with saidfirst optical axis, optically coupling said first imaging system withsaid second imaging system so that an image of said first imaging systemis an object of said second imaging system and an image of said secondimaging system is an object of said first imaging system, including saidactive mirror in said first imaging system or said second imagingsystem, reflecting a pump light beam multiple times between said firstimaging system and said second imaging system, focusing said pump lightbeam to be an object of said first imaging system, and striking saidactive mirror multiple times with said pump light beam when said beam isreflected between said first imaging system and said second imagingsystem.
 28. A method according to claim 27 including: forming an anglewith said first optical axis and said second optical axis, wherein saidobject of said first imaging system being said image of said secondimaging system and said object of said second imaging system being saidimage of said first imaging system are substantially unchanged when saidpump light beam is reflected between said first imaging system and saidsecond imaging system, and wherein said object of said first imagesystem or said second image system is at said active mirror.
 29. Amethod according to claim 27 including: arranging said first opticalaxis and said second optical axis parallel and separating them by adistance, wherein said object of said first imaging system being saidimage of said second imaging system and said object of said secondimaging system being said image of said first imaging system move whensaid pump light beam is reflected between said first imaging system andsaid second imaging system, wherein said pump light beam is collimatedby a lens of said first imaging system and reflected by said activemirror disposed at a focal plane of said lens, and wherein saidcollimated pump light beam strikes and is reflected by said activemirror at substantially unchanged position.
 30. A method according toclaim 27: arranging said first optical axis and said second optical axisparallel and separated by a distance, wherein said object of said firstimaging system being said image of said second imaging system and saidobject of said second imaging system being said image of said firstimaging system are substantially unchanged when said pump light beam isreflected between said first imaging system and said second imagingsystem, and wherein said object of said first image system or saidsecond image system is at said active mirror.